Avast is another one that is pretty good for free. No annoying pop-ups either reminding you to buy the full version like some. And, it's light on the resources.http://www.avast.com/index
One of the drawbacks was it's slow scan time. They've fixed that where it caches files it's already scanned and deemed safe. I've seen where my full scan over all my drives (750 gigs) took about 2.5 hours. Now, it takes about 1 hour and 15 minutes.
My quick scan takes about 7-9 minutes.
Still not the fastest but nowhere near the slow times it used to get.